1. Field of the Invention
This invention relates to methods for entrapment of substantial quantities of aqueous fluids.
2. Description of the Art Practices
It is known that many compositions will absorb aqueous fluids to some degree. Within the last few years, however, it has become possible to develop products which absorb several hundred times their own weight of pure distilled water. When free ions are present in the water, such as from sodium chloride, however, the overall absorbency of the compositions is significantly lessened. This presents some difficulty when the fluid to be absorbed is urine, menstrual discharge, or other bodily fluids such as collect in wounds which have high ionic strength.
Thus, a requirement for the practical application of a water absorbent polymer is that the material retain a high capacity for the absorbence of the fluid, even in the presence of ions such as sodium.
A second problem which is observed in the development of such water absorbent products is that while having a high capacity to absorb the aqueous fluid they do not do so because of the surface wetting phenomena of the particles. That is, under the best conditions, several hundred times the weight of the water absorbent polymer will be absorbed when using deionized water. The problem is, however, that this is done under conditions of ideal mixing. However, when dealing with a bandage, cetamenial tampon, or a diaper, the water absorbent polymer is fixed in its position with relation to the aqueous fluid to be absorbed. Therefore, when surface wetting occurs, it is observed that the first portion of the aqueous fluid to contact the water absorbent polymer is rapidly absorbed on the surface thereof. However, this first portion of the aqueous fluid does, by a mechanism not fully understood, block the remaining flow of the aqueous fluid into the interior of the water absorbent polymer.
It has been proposed in U.S. Pat. No. 4,043,952, issued to Ganslaw, et al, on Aug. 23, 1977 to modify only the surface of a water absorbent polymer to improve the overall absorbence capability of the water absorbent polymer. This method of effecting the surface characteristic is said, by the author of the foregoing reference, to improve the dispersability of the water absorbent polymer. That is, by altering the surface of the polymer with metallic ions, it is stated to be possible to increase the water carrying capacity of the polymer. It is also stated in the Ganslaw, et al. patent that any polyvalent cation may be employed in the surface treating, but with the express requirement that the linkage density at the particle surface must be greater than the linkage density at the particle interior. By greater linkage density at the particle surface, it is meant that the number of ionic linkages on the surface of the polymer molecule are in greater number than in the respective interior portion of the molecule.
While modifying the absorbent characteristics of a polymer may work in some instances, it can not be stated to be a predictable. That is, even though the foregoing reference states that any polyvalent cation may be employed, it has been observed that, with certain water absorbent polymers, the use of certain polyvalent cations renders the water absorbent polymer substantially inactive. That is, not only does the product not wick (absorb rapidly), it loses substantial capability to absorb water when compared to the basic polymer which has not been reacted with the polyvalent cation.
The present invention is particularly concerned with hydrolyzed starch graft copolymers which exhibit substantial capacity to absorb several hundred times their own weight of water with rapid wicking. The development of the original (unmodified) compositions was carried out by the Northern Regional Research Laboratory of the U.S. Department of Agriculture. The basic hydrolyzed starch polyacrylonitrile graft copolymer is produced through free-radical polymerization followed by base hydrolysis.
In the free-radical polymerization process, starch, either gelatinized or ungelatinized, is exposed to a catalyst such as ceric ammonium nitrate which is capable of generating free-radicals in the starch chain. The free-radical generation may also be accomplished through the use of gamma radiation. The overall result is to add polyacrylonitrile chains to the starch free-radicals thus forming the copolymer. In this regard, a description of the processes may be found in U.S. Pat. No. 3,935,099 issued to Weaver, et al. In this reference it is suggested that the ratio of the starch to the polyacrylonitrile should be on a molar basis from about 1:1.5 to 1:9. The variations in the molar ratio of the components of the copolymer are not apparently critical to the practice of the present invention. The resulting copolymer which contains nitrile groups is then hydrolyzed using a strong base such as sodium or potassium hydroxide in the presence of water to convert the nitrile groups to carboxyls and amides. It is observed, in carrying out the hydrolysis step that an extremely strong base is required and materials such as ammonium hydroxide are not suitable for the conversion of the nitrile groups.
The hydrolyzed material may then be dried by any convenient method, such as tumbling or vacuum drying. It is often convenient, prior to drying the absorbent polymer, to wash it with an alcohol to remove any excess ions present, i.e. excess caustic, to substantially increase the absorbency of the composition.
It is noted, however, that the foregoing is accomplished using ideal conditions with respect to dispersability and absorbent capacity. That is, the product, when placed in a diaper or bandage as previously suggested, may well suffer from a lack of absorbency due to the surface wetting phenomenon. It has previously been suggested that such hydrolyzed starch polyacrylonitrile graft copolymers may be modified through treatment with formaldehyde to form a cross-linked product which has increased water absorption capability. The difficulty, of course, involved in this process involves the handling of formaldehyde which, while used in substantial industrial operations, is nonetheless still a hazardous material. Moreover, extreme care must be taken to remove as much unreacted formaldehyde as possible from the copolymer as it is to be used in contact with the human body. Thus, there is a need to obtain a highly water absorbent composition made from a hydrolyzed starch polyacrylonitrile graft copolymer having increased absorbence and wicking without any deleterious side effects.
The present invention has, through treatment of the hydrolyzed starch polyacrylonitrile graft copolymer with aluminum ions, obtained a product having a substantially enhanced degree of wicking and overall improved absorbence.
Through the specification and claims, percentages and ratios are given by weight unless otherwise indicated and temperatures are in degrees Celsius.